1. Field of the Invention
The present invention relates to a current drive circuit. For example, the present invention relates to a current drive circuit suitable for driving a current driving element such as a laser diode that is mounted in an optical disk device and is required to be supplied with a stable drive current.
2. Description of Related Art
In an optical information processor, a laser diode (hereinafter, referred to also as “LD”) is widely used as a light source. For example, a laser diode is used as a light source for an optical head in an optical disk device. The laser diode is driven by a current drive circuit. Accordingly, it is necessary to supply a stable drive current to the laser diode regardless of a power supply voltage variation. As a current drive circuit of this type, a current-mirror type circuit is widely employed.
In general, a ratio of the magnitude of currents flowing through two metal-oxide-semiconductor field-effect transistors (MOSFETs) constituting a current mirror, that is, a mirror ratio is determined based on a ratio of the size (channel width W/channel length L) of two MOSFETs. However, it is known that a current flowing through the MOSFETs is affected by a source-drain voltage VDS due to a channel length modulation effect. Accordingly, if the effect is not taken into account, there is a fear that, even when the W/L ratio is correctly set, a stable drive current cannot be obtained because of the power supply voltage variation due to noise or the like.
Japanese Unexamined Patent Application Publication No. 2005-101154 and Japanese Unexamined Patent Application Publication No. 2006-114895, which is filed as a divisional application thereof, each discloses a circuit configuration for stabilizing the drive current against the power supply voltage variation. FIG. 4 is a circuit diagram shown in FIG. 1 of Japanese Unexamined Patent Application Publication No. 2005-101154.
The circuit shown in FIG. 4 includes a laser diode LD1, an output current setting current source IS, an LD output terminal T1, an output MOSFET M1, a MOSFET M2, an output switch SW1, a MOSFET M3, a MOSFET M4, a first dummy LD LD2, a MOSFET M5, a MOSFET M6, an output switch for the first dummy LD SW2, a MOSFET M7, a MOSFET M8, a second dummy LD LD3, a MOSFET M9, a MOSFET M10, a MOSFET MN1, a MOSFET M12, a MOSFET M13, a MOSFET M14, and a correction amount detection amplifier AMP.
In this case, the MOSFET M1 and the MOSFET M2 constitute a current mirror CM1.
The MOSFET M5 and the MOSFET M6 constitute a current mirror CM2.
The MOSFET M9, the MOSFET M10, the MOSFET MN1, and the MOSFET M12 constitute a cascode current mirror CM3.
The MOSFET M14 and the MOSFET M13 constitute a current mirror CM4.
The MOSFET M14 and the MOSFET M8 constitute a current mirror CM5.
Further, the MOSFET M14 and the MOSFET M4 constitute a current mirror CM6.
Next, a description is given of operations of the circuit shown in FIG. 4.
First, a current I1 proportional to a desired current IOUT1, which is caused to flow through the laser diode LD1, flows from the output current setting current source IS. When the output ON/OFF switch SW1 is turned on, the current is supplied as the current IOUT1 to the laser diode LD1 via the current mirror CM6, the current mirror CM1, and the LD output terminal T1, whereby the laser diode LD1 emits light. In this case, it is assumed that no current flows through the MOSFET M3.
Assuming that the power supply voltage varies due to an effect of noise or the like, a terminal voltage VLD of the laser diode LD1 is substantially constant, while a voltage V of the power supply VDD varies. In other words, a voltage VDS1 applied between a drain and a source of the MOSFET M1 varies. As a result, owing to the channel length modulation effect of the MOSFET, a current flowing through the MOSFET M1 fluctuates, which causes a problem. In this case, it is necessary to supply a large current to the current mirror CM1, which is formed of the MOSFET M1 and the MOSFET M2, in a state where a sufficient amount of the voltage VDS1 cannot be supplied. As a result, the current mirror CM1 cannot be implemented with a cascode configuration.
Next, a description is given of a circuit operation for suppressing the current fluctuation.
First, a current proportional to the current I1, which flows through the MOSFET M14, flows through the MOSFET M13 that constitutes the current mirror CM4 with the MOSFET M14. The current is supplied as a current I3 to the second dummy LD LD3 via the cascode current mirror CM3.
On the other hand, the current proportional to the current I1, which flows through the MOSFET M14, also flows through the MOSFET M8 that constitutes the current mirror CM5 with the MOSFET M14. The current is supplied as a current I4 to the first dummy LD LD2 via the switch SW2, which is constantly turned on, and the current mirror CM2.
In this case, it is assumed that the first dummy LD LD2 and the second dummy LD LD3 have the same characteristics. In a case where the power supply voltage is constant at the voltage V, when the current flowing through the current mirrors CM2, CM3, CM4, and CM5 and the MOSFET M7 is set so as to satisfy I3=I4, an anode voltage of the first dummy LD LD2 becomes equal to an anode voltage of the second dummy LD LD3. As a result, a potential difference between an inverting input terminal and a non-inverting input terminal of the correction amount detection amplifier AMP is eliminated.
Consideration is made of a case where the power supply voltage varies under the set conditions, for example, a case where the voltage of the power supply VDD increases. The current I3 supplied to the second dummy LD LD3 is substantially constant because the current mirror CM3 has a cascode configuration. On the other hand, since the current mirror CM2 is not implemented with the cascode configuration and a voltage VD33 applied between a drain and a source of the MOSFET M5 is high, the current I4 supplied to the first dummy LD LD2 has a large current value owing to the channel length modulation effect, whereby I3<I4.
Accordingly, the anode voltage of the first dummy LD LD2 becomes higher than the anode voltage of the second dummy LD LD3, and an output voltage of the correction amount detection amplifier AMP decreases. Thus, the current flowing through the MOSFET M7 becomes smaller, with the result that I3=I4. Therefore, the current flowing through the first dummy LD LD2 remains constant regardless of power supply variations.
In contrast, when the power supply voltage decreases, the anode voltage of the first dummy LD LD2 becomes lower than the anode voltage of the second dummy LD LD3, and the output voltage of the correction amount detection amplifier AMP increases. Thus, the current flowing through the MOSFET M7 becomes larger, with the result that I3=I4. Accordingly, the current flowing through the first dummy LD LD2 remains constant regardless of power supply variations.
A correction current flowing through the MOSFET M7 is also caused to flow through the MOSFET M3 in a similar manner, whereby the current flowing through the laser diode LD1 can be set constant regardless of power supply variations.
However, in the circuit configuration disclosed in each of Japanese Unexamined Patent Application Publication No. 2005-101154 and Japanese Unexamined Patent Application Publication No. 2006-114895, the correction amount detection amplifier, the dummy laser diode, and the like are necessary. Accordingly, there arises a problem in that the circuit is complicated and the chip size is increased, which leads to an increase in costs.